Getting on a dog’s good side might seem as easy as having a pocket full of treats and knowing the trick to a good belly rub, but our four-legged friends aren’t always easy to please. They’re quick to judge a person’s character, and there are some people they simply don’t like.

It could be a specific person in the dog’s family, a friend of their owner’s, or a random person they meet on the street—but dogs know a foe when they see one. They might growl if the person comes close or simply turn tail and disappear. It seems random, but it isn’t as mysterious as you think. Here are a few reasons why your dog doesn’t like certain people.

#1 – Tone of Voice

Dogs might not be fluent in your language, but they’re experts at picking up the tone of voice. A scientific study published in 2016 found that dogs’ brains react based on the tone of voice of the person talking to them.

In the study, the reward centers in the dogs’ brains became activated when the person used a high-pitched, happy voice. The dogs were glad to greet the happy-sounding person, but they reacted negatively or ignored people who spoke with deep or angry-sounding voices.

#2- Body Language

While your dog is assessing a person’s tone of voice, they’re also observing their body language. Dogs depend on body language to help fill in communication gaps. The trouble comes when comparing the way humans perceive specific body movements to how dogs interpret those same signs.

Eye contact, for example, means different things to different species. Between humans, someone who doesn’t make direct eye contact is perceived as shifty, untruthful, or suspicious. In a dog’s world, however, direct eye contact is rude and even threatening. Giving “soft eyes,” or looking slightly to the side, is a sign of respect or deference. Bending over the dog, making wide gestures with your arms, erratic movements, and forcing a dog into a hug are all bad body moves dogs don’t appreciate.

#3 – How Someone Interacts With Other People

A comparative psychologist at Kyoto University performed a study to determine whether certain animals are capable of making social evaluations in the same way humans do. He wanted to know if dogs could tell when a person was being rude to another person and if that knowledge would affect their opinion of the person.

He did a test where a dog watched their owner struggle to open a container. The owner then asked another person for help. Sometimes the person helped, and sometimes they refused. After each interaction, the dog was given the choice of whether to accept attention from the other person or ignore them. On turns when the person refused to help the dog’s owner, the dog was more likely to show signs of not liking the rude person. The study shows if a person in your life is regularly rude to you, your dog will decide for themselves they don’t like them. Dogs are Team Owner all the way!

#4 – Smell

Everyone knows a dog’s sense of smell is incredibly powerful. Their first move when being introduced to a new dog or person is to give them a good sniff. If they like what they smell and the interaction goes well—BAM, best friends for life. But if they get a whiff of something intimidating, confusing, or downright repugnant, they’ll follow their nose to better smells.

Dogs that dislike other dogs will often avoid people who smell like unknown canines. Other smells dogs typically don’t like include citrus, vinegar, mothballs, and rubbing alcohol.

#5 – Past Trauma

Rescue dogs with histories of abuse and abandonment don’t forget about their traumatic experiences. Even when they move on and are adopted by loving families, the suffering they went through will often stick with them. In some cases, they develop fears and mistrust toward people who remind them of their difficult pasts.

If a rescue regularly cowers around adult men but turns toward women for comfort, there’s a good chance they once suffered abuse at the hand of a man. Gender, hair color, height, race, general appearance—the dog might respond negatively to anyone who reminds them of past pain.